A propeller creates a thrust force out of the supplied power. The magnitude of this force is not constant for a given propeller, but depends on the velocity of the incoming air and the rotational velocity of the propeller itself. Thus tests of propellers usually cover a wide regime of operating conditions.
Propellers having the same shape, but scaled by a size factor behave similar. In order to make a comparison of propellers of different size easier, aerodynamicists try to get rid of the units. Then it is possible to use the results of a small scale wind tunnel model to predict the performance of a full scale propeller. Similar to airfoils and wings, the performance of propellers can be described by dimensionless (normalized) coefficients. While an airfoil can be characterized by relations between angle of attack, lift coefficient and drag coefficient, a propeller can be described in terms of advance ratio, thrust coefficient, and power coefficient. The efficiency, which corresponds to the L/D ratio of a wing, can be calculated from these three coefficients. These coefficients are helpful for the comparison of propellers of differing diameters, tested under different operating conditions.
Depending on the country where you live, there may be different coefficients in use. All coefficients used here, are based on the publications of the NACA, which defined the following coefficients:
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where
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It should be noted, that the definition of the efficiency includes the velocity v. Thus the efficiency goes to zero when the flight speed approaches zero - of course, this does not mean, that the thrust goes to zero. Usually the power and thrust coefficients are plotted versus the advance ratio. The efficiency of a pylon racing model propeller, as calculated from these coefficients, is show below.
A typical plot of efficiency versus advance ratio v/nD of a propeller for
model airplanes.
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